He was charged with contempt of congress, therefore if he still fails to show the he is entitled to stand trial on the charge of contempt. Therefore the issuing body (the house committee) is forced to provide his defense with any and all evidence on why he was to appear and that included prepared questions....Congress can't make you appear on a whim, certain criteria must be met first.
.....a congressional committee must meet three requirements for its subpoenas to be "legally sufficient." First, the committee's investigation of the broad subject area must be authorized by its chamber; second, the investigation must pursue "a valid legislative purpose" but does not need to involve legislation and does not need to specify the ultimate intent of Congress; and third, the specific inquiries must be pertinent to the subject matter area that has been authorized for investigation.
That said the CoC law is so screwy (in favor of congress) as to appear a pretzel so it's a crap-shoot.